This invention relates to copying apparatus which automatically controls the copy quality according to the density of copied documents.
In conventional copying apparatus, an operator operates a dial or lever, or selects one of several copy buttons according to the nature of a copied document. This usually adjusts the exposure time of the photosensitive material, or adjusts a bias voltage value of a developing device to obtain a good copy. However, this type of apparatus requires manual operation of the dial or lever, or selection of a button according to the density of the document, in addition to manually pressing the copy start button. Furthermore, the above apparatus has a disadvantage in that copies of poor quality sometimes result, due to the visual perception of the operator. Therefore, copying apparatus in which the density of a document is detected and automatically used to adjust the exposure time or the bias voltage of the developing device have recently been developed.
In the latter copying apparatus, there are several methods for detecting density, such as shown in Japanese patent disclosures Nos. 53-93834, 53-93835, 53-93836. One method is to detect minimum density (and/or maximum density) of a document by detecting the light reflected from the document. However, this method is vulnerable to electrical noises or mechanical vibrations. Although copies having high picture quality generally can be obtained by the above method because the darkest and lightest parts are detected, it is often difficult to obtain high quality copies because true density values are not detected, even if the minimum density is above a predetermined density value.
Another known method determines the density of a document by detecting the light reflected from the document to control copy quality. However, the disadvantage of this method is that the control signal changes in response to the ratio of the area of the dark or black part to the light or white part even if density values in the black part and white part are the same in two documents.